Image noise reduction

ABSTRACT

A reduced noise image can be formed from a set of images. One of the images of the set can be selected to be a reference image and other images of the set are transformed such that they are better aligned with the reference image. A measure of the alignment of each image with the reference image is determined. At least some of the transformed images can then be combined using weights which depend on the alignment of the transformed image with the reference image to thereby form the reduced noise image. By weighting the images according to their alignment with the reference image the effects of misalignment between the images in the combined image are reduced. Furthermore, motion correction may be applied to the reduced noise image.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 15/068,899filed Mar. 14, 2016, which claims foreign priority from United KingdomApplication No. 1504316.9 filed Mar. 13, 2015.

BACKGROUND

Cameras are used to capture images. Often images are noisy in the sensethat there is some image noise present in the image. The image noise maybe random (or pseudo-random) such that there is little or no correlationbetween the image noise of two different images of the same scene. Inthe context of this description, image noise is an unwanted signal whichis present in an image resulting from the image capture process, and maybe produced, for example, by a sensor and/or by circuitry of a camerawhich captures the image.

Since there is often little or no correlation between the image noise oftwo different images of the same scene, the image noise may be reducedby combining a sequence of two or more images captured in quicksuccession of the same scene. Combining the images will reduce theeffect of random fluctuations in each individual image resulting fromthe image capture process. For example, at each pixel position, thepixel values for the different images may be averaged to determine thepixel values of the combined image. The combined image is a reducednoise image.

Since, the images which are combined are captured at different timeinstances there may be some motion of objects in the scene between thetimes at which different images are captured. Furthermore, there may besome movement of the camera between the times at which different imagesare captured. In particular, if a user is holding a camera while itcaptures a sequence of images then it is very likely that there will besome camera movement between the times at which different images arecaptured. The motion between the images which are combined to form thereduced noise image may cause some geometric misalignment between theimages, which in turn may introduce some blur into the reduced noiseimage. There are various types of “alignment” between images, such asgeometric alignment, radiometric alignment and temporal alignment. Thedescription herein considers geometric alignment of images which isrelevant for handling motion between the images, and the term“alignment” as used herein should be understood to be referring to“geometric alignment”. Misalignment between the images causes problemswhen it comes to combining images in order to reduce noise. Furthermore,movement of the camera while an image is being captured may introducemotion blur into the image which can reduce the sharpness of the image.

SUMMARY

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter.

There is provided a method of forming a reduced noise image using a setof images, the method comprising: applying respective transformations toat least some of the images of the set to bring them closer to alignmentwith a reference image from the set of images; determining measures ofalignment of the respective transformed images with the reference image;determining weights for one or more of the transformed images using thedetermined measures of alignment; and combining a plurality of imagesincluding said one or more of the transformed images using thedetermined weights to form a reduced noise image.

There is provided a processing module for forming a reduced noise imageusing a set of images, the processing module comprising: alignment logicconfigured to: apply respective transformations to at least some of theimages of the set to bring them closer to alignment with a referenceimage from the set of images; and determine measures of alignment of therespective transformed images with the reference image; and combininglogic configured to: determine weights for one or more of thetransformed images using the determined measures of alignment; andcombine a plurality of images including said one or more of thetransformed images using the determined weights to form a reduced noiseimage.

There is provided a method of transforming a first image to bring itcloser to alignment with a second image, the method comprising:implementing a multiple kernel tracking technique to determine positionsof a set of candidate regions of the first image based on a similaritybetween a set of target regions of the second image and the set ofcandidate regions of the first image, wherein the target regions of thesecond image are respectively positioned over the positions of apredetermined set of points of the second image; using at least some ofthe determined positions of the set of candidate regions to initialize aLucas Kanade Inverse algorithm; using the Lucas Kanade Inverse algorithmto determine a set of points of the first image which correspond to atleast some of the predetermined set of points of the second image;determining parameters of a transformation to be applied to the firstimage based on an error metric which is indicative of an error between atransformation of at least some of the determined set of points of thefirst image and the corresponding points of the predetermined set ofpoints of the second image; and applying the transformation to the firstimage to bring it closer to alignment with the second image.

There is provided a processing module for transforming a first image tobring it closer to alignment with a second image, the processing modulecomprising alignment logic which comprises: multiple kernel trackinglogic configured to implement a multiple kernel tracking technique todetermine positions of a set of candidate regions of the first imagebased on a similarity between a set of target regions of the secondimage and the set of candidate regions of the first image, wherein thetarget regions of the second image are respectively positioned over thepositions of a predetermined set of points of the second image; LucasKanade Inverse logic configured to use a Lucas Kanade Inverse algorithmto determine a set of points of the first image which correspond to atleast some of the predetermined set of points of the second image,wherein the positions of at least some of the set of candidate regionsdetermined by the multiple kernel tracking logic are used to initializethe Lucas Kanade Inverse algorithm; and transformation logic configuredto: (i) determine parameters of a transformation to be applied to thefirst image based on an error metric which is indicative of an errorbetween a transformation of at least some of the determined set ofpoints of the first image and the corresponding points of thepredetermined set of points of the second image, and (ii) apply thetransformation to the first image to bring it closer to alignment withthe second image.

There may also be provided computer readable code adapted to perform thesteps of any of the methods described herein when the code is run on acomputer. Furthermore, computer readable code may be provided forgenerating a processing module according to any of the examplesdescribed herein. The computer code may be encoded on a computerreadable storage medium.

The above features may be combined as appropriate, as would be apparentto a skilled person, and may be combined with any of the aspects of theexamples described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

Examples will now be described in detail with reference to theaccompanying drawings in which:

FIG. 1 is a schematic diagram of a processing module for forming areduced noise image;

FIG. 2 is a flow chart of a method of forming a reduced noise image;

FIG. 3 is a graph showing the values of sharpness indications for a setof images;

FIG. 4 is a flow chart of a method of determining point correspondencesbetween two images;

FIG. 5 shows a set of regions within an image used for a multiple kerneltracking technique and a corresponding set of regions within the imageused for a Lucas Kanade Inverse algorithm;

FIG. 6 is a graph showing the values of misalignment parameters for aset of images;

FIG. 7a shows an example of an average of a set of images when there ismotion in the scene;

FIG. 7b shows a binary motion mask indicating areas of motion in the setof images;

FIG. 7c shows a modified motion mask;

FIG. 7d shows a smoothed motion mask;

FIG. 8 shows an example of a reference image, a reduced noise image anda motion-corrected reduced noise image;

FIG. 9 is a schematic diagram of a computer system in which a processingmodule for forming a reduced noise image is implemented; and

FIG. 10 shows an integrated circuit manufacturing system for generatingan integrated circuit embodying a graphics processing system.

The accompanying drawings illustrate various examples. The skilledperson will appreciate that the illustrated element boundaries (e.g.,boxes, groups of boxes, or other shapes) in the drawings represent oneexample of the boundaries. It may be that in some examples, one elementmay be designed as multiple elements or that multiple elements may bedesigned as one element. Common reference numerals are used throughoutthe figures, where appropriate, to indicate similar features.

DETAILED DESCRIPTION

In examples described herein, a reduced noise image is formed using aset of images. One of the images of the set may be selected to be areference image and other images of the set are transformed such thatthey are better aligned with the reference image. At least some of theimages can then be combined using weights which depend on the alignmentof the transformed image with the reference image to thereby form thereduced noise image. By weighting the images according to theiralignment with the reference image the effects of misalignment betweenthe images in the combined image are reduced.

Furthermore, in examples described herein, point correspondences betweena first image of a set of images and a second image (e.g. a referenceimage) of the set of images can be determined by implementing a multiplekernel tracking (MKT) technique to determine positions of a set ofcandidate regions of the first image, and using the determined positionsto initialize a Lucas Kanade Inverse (LKI) algorithm. The LKI algorithmcan then be used to determine a set of points of the first image whichcorrespond to at least some of a predetermined set of points of thesecond image. These point correspondences can then be used to determineparameters of a transformation to be applied to the first image to bringit closer to alignment with the second image. It is noted that the MKTtechnique gives a global alignment which includes alignment parametersdescribing an alignment for the full image, and then this globalalignment is used to determine the initial positions for use in the LKIalgorithm which then obtains a local alignment to determine the pointcorrespondences. As described in more detail below, the use of amultiple kernel tracking technique to initialize a Lucas Kanade Inversealgorithm solves some problems which can sometimes be encountered with aLucas Kanade Inverse algorithm. For example, without a sufficientlyaccurate initialization, the Lucas Kanade Inverse algorithm may fail toconverge on an accurate solution. The use of a multiple kernel trackingtechnique can provide a sufficiently accurate initialization for theLucas Kanade Inverse algorithm even if the point correspondences involvea large shift in position and even if there are affine transformations,such as rotations, between the images. Furthermore, the Lucas KanadeInverse algorithm does not perform well in flat areas of an imagebecause the algorithm uses gradients to converge on a solution. Amultiple kernel tracking technique includes the calculation of featurehistograms which can be used to indicate whether a region is flat andshould therefore be discarded such that it is not used when implementingthe Lucas Kanade Inverse algorithm.

In more detail, in examples described herein, the candidate images (i.e.the images other than the reference image) are warped back to thereference image using the MKT parameters, such that any region from acandidate image should be close to the corresponding region of thereference image. The LKI algorithm can then use the same regions thatwere used when performing the MKT, because some information is alreadycomputed for them (e.g. as described below, an intensity histogram maybe computed for a region which can be used to determine if the region isflat or not). The MKT technique can include scaling and rotationfunctions, so warping the full candidate image back to the referenceimage can have some accuracy advantages, since the LKI algorithmdescribed herein does not include scaling or rotation functions. The LKIalgorithm described herein does not include scaling or rotationfunctions because it operates on small regions, so allowing scaling androtations would introduce too many degrees of freedom for the smallregion thereby resulting in errors. So, the use of the MKT techniquetakes scaling and rotation into account, such that the LKI algorithmdoes not need to, and the method still has tolerance to rotations andscaling. It is noted that the point correspondences obtained by the LKIalgorithm provide projective transformations which may include scalingand rotation. Projective transformations are not estimated on the MKTstep in the examples described herein because the MKT technique wouldbecome unstable due to too many degrees of freedom. The MKT techniquedescribed herein has four degrees of freedom (x, y, scale, angle) and aprojective transformation has eight degrees of freedom.

Embodiments will now be described by way of example only.

FIG. 1 shows a processing module 100 which is configured to receive aset of images and to form a reduced noise image using the set of images.Furthermore, in the example shown in FIG. 1, the processing module 100is configured to apply motion correction such that the image which isoutput from the processing module 100 is a motion-corrected, reducednoise image. The processing module 100 comprises selection logic 102,alignment logic 104, combining logic 106 and motion correction logic108. The alignment logic 104 comprises point correspondence logic 110,transformation logic 112 and alignment measuring logic 114. The pointcorrespondence logic 110 comprises multiple kernel tracking logic 116and Lucas Kanade Inverse logic 118. The processing module 100, and itslogic blocks, may be implemented in hardware, software or a combinationthereof.

The operation of the processing module 100 is described with referenceto the flow chart shown in FIG. 2. In step S202 the processing module100 receives a set of images. To give some examples, the images may bereceived from an image sensor, from some other processing module or froma memory which may be implemented on the same device (e.g. camera,smartphone, tablet, etc.) as the processing module 100. The images ofthe set of images are similar in the sense that they are substantiallyof the same scene. For example, the set of images may be captured inquick succession, e.g. with a camera operating in a burst mode such thata plurality of images (e.g. 24 images) are captured over a short timeperiod (e.g. 3 seconds). The numbers given herein are given by way ofexample and may be different in different implementations. The set ofimages may comprise frames of a video sequence. The set of images arereceived at the selection logic 102.

As a very brief overview of the noise reduction method implemented bythe processing module 100:

-   -   the selection logic 102 selects a reference image from the set        of images based on the sharpness of the images, and discards        blurry images (steps S204 to S208);    -   the alignment logic 104 transforms images such that they more        closely align with the reference image, and discards those which        are highly misaligned (steps S210 to S218);    -   the combining logic 106 combines images to form a reduced noise        image (steps S220 and S222); and    -   the motion correction logic 108 corrects artifacts in the        reduced noise image which are produced by motion between the        images (steps S224 and S226).

These processes are described in more detail below.

In step S204 the selection logic 102 determines sharpness indicationsfor the images. It is noted that a camera capturing the images may beimplemented in a handheld device and, as such, some of the images may beblurry due to motion of the camera. Blur caused by motion of the camerais not normally a desired effect. Therefore, in step S206, if thedetermined sharpness indication for an image is below a sharpnessthreshold then the image is discarded.

As an example, the sharpness indications may be sums of absolute valuesof image Laplacian estimates for the respective images. The imageLaplacian is a good indicator of the presence of high frequencies in animage, and a blurry image usually has less high frequency energy. TheLaplacian, L(I_(i)(x, y)), at a pixel position (x,y) of the image I_(i),is the 2^(nd) derivative of the image at that pixel position and isgiven by the equation:

${{L\left( {I_{i}\left( {x,y} \right)} \right)} = {\frac{\partial^{2}I_{i}}{\partial^{2}x} + \frac{\partial^{2}I_{i}}{\partial^{2}y}}},$

where I_(i)(x, y) is the image pixel value at the location (x, y) and Lis the Laplacian operator.

Computing the Laplacian is a simpler operation than computing themagnitude of the gradients. The second derivatives (which are calculatedfor the Laplacian) are more sensitive to noise than the magnitude of thegradients, so in some examples the magnitude of the gradients may beused to determine the sharpness indications, but in the examplesdescribed in detail herein the Laplacian is used due to its simplicityand an assumption can be made that the noise will be approximately thesame for each image. For example, the Laplacian may be estimated byfiltering the image with a suitable filter.

The sharpness indication for an image, i, is denoted φ_(i), and is thesum of the absolute values of the image Laplacian over all of the pixelpositions of the image, such that:

φ_(i)=Σ_(xy) |L(I _(i)(x, y))|,

The sharpness indication of an image is a measure of the sharpness (orconversely the blurriness) of the image. The sharpness threshold may bedetermined using the mean, μ(φ), and standard deviation, σ(φ), of theset of sharpness indications, φ, for the set of images, where φ={φ₁, . .. , φ_(N)} for a set of N images. For example, the threshold may be setat μ(φ)−ε₁σ(φ) (where as an example ε₁ may be in the range 1.1≤ε₁≤1.4),wherein an image is discarded if its sharpness indication is below thisthreshold. That is, the image, i, is discarded in step S206 if:

φ_(i)<μ(φ)−ε₁σ(φ).

As an example, FIG. 3 shows a graph of sharpness indications 302 _(i)for a set of ten images (i=0 . . . 9). In this example, the sharpnessthreshold is shown by the dashed line 304. Images 6 and 7 have sharpnessindications 302 ₆ and 302 ₇ which are below the sharpness threshold 304.Therefore images 6 and 7 are discarded in step S206 because they aredetermined to be too blurry. The sharpness indications 302 of the otherimages are above the sharpness threshold 304 and as such those otherimages are not discarded in step S206. It is noted that in some otherexamples step S206 might not be performed. That is, in some examples,images are not discarded based on their sharpness. This may help tosimplify the process, but may result in more blurriness appearing in thefinal image.

In step S208, based on the sharpness indications 302, the selectionlogic 102 selects the sharpest image from the set of images to be thereference image. Therefore, in the example shown in FIG. 3, image 5 isselected to be the reference image because its sharpness indication 302₅ is higher than the sharpness indications 302 of the other images inthe set of ten images. Selecting the sharpest image as the referenceimage is beneficial to the rest of the method described below. Forexample, it is easier to determine alignment to a sharp image than todetermine alignment to a blurry image. In other examples, a referenceimage could be selected using different criteria, e.g. a combination ofdifferent criteria. For example, a reference image could be selectedbased on the content of the images, e.g. the image from the set ofimages in which the greatest number of people are smiling or in whichthe greatest number of people have their eyes open and/or are looking atthe camera may be selected as the reference image. In general, the“best” image may be selected as the reference image, but the criteriawhich determine which image is considered to be the best may bedifferent in different examples.

The images which have not been discarded (e.g. images 0 to 5 and 8 and 9in the example shown in FIG. 3) are passed from the selection logic 102to the alignment logic 104. In steps S210 to S214 the alignment logic104 determines and applies a respective transformation to each of theimages (other than the reference image and the images discarded by theselection logic 102) to bring them closer to alignment with thereference image. In the examples described below, the transformation foran image, I_(i), is represented as a homography, H_(i). The homographyH_(i) is a matrix which is determined with the aim of satisfying theequation:

x_(i)=H_(i)x_(r),

where x_(i) is a set of points of the image I_(i) and x_(r) is acorresponding set of points of the reference image I_(r). So in order todetermine the parameters of the transformation (i.e. the components ofthe homography matrix H_(i)) point correspondences are first determined,i.e. it is determined which points of the image I_(i) correspond to atleast some of the set of points x_(r) of the reference image I_(r). Theset of points x_(r) of the reference image I_(r) is a predetermined setof points, and may for example comprise points of a uniform lattice.

Therefore, in step S210, the point correspondence logic 110 determines,for each of the images to which a transformation is to be applied, a setof points x_(i) which correspond to the predetermined set of pointsx_(r) of the reference image I_(r). In the example described herein, theset of points x_(i) is determined using the Lucas Kanade Inverse (LKI)algorithm. Furthermore, the LKI algorithm is initialized using theresults of a multiple kernel tracking (MKT) technique.

Details of step S210 are shown in the flow chart of FIG. 4. Inparticular, step S210 includes steps S402, S404 and S406. In step S402the MKT logic 116 implements a MKT technique to determine positions of aset of candidate regions of the image I_(i) based on a similaritybetween a set of target regions of the reference image I_(r) and the setof candidate regions of the image I_(i).

FIG. 5 represents an image I_(i), denoted 502. The positions of thepredetermined set of points of the reference image create a uniformlattice over at least part of the image 502, and FIG. 5 shows thesepoints (one of which is denoted with reference numeral 504). In thisexample the lattice is a 5×7 lattice of points 504 but in other examplesa different arrangement of predetermined points may be used, e.g. a10×10 lattice. The circles 506 shown in FIG. 5 represent the candidateregions for which the positions are determined by the MKT logic 116 instep S402. The squares 508 shown in FIG. 5 represent candidate regionsused by the LKI algorithm as described below.

In the MKT technique, the candidate regions 506 are compared to targetregions of the reference image I_(r). The circles 506 in FIG. 5 aremerely illustrative, and the regions could have any suitable shape, e.g.the target regions may be blocks of 31×31 pixels of the reference image,positioned over (e.g. centred on) the positions of the points 504 fromthe predetermined set of points of the reference image I_(r).

Multiple kernel tracking techniques are known in the art, for example asdescribed in “Multiple kernel tracking with SSD” by Hager, Dewan andStewart, IEEE Conference on Computer Vision and Pattern Recognition,2004, pp790-679. As such, for conciseness, an in depth explanation of amultiple kernel tracking technique is not provided herein. However, as ahigher-level explanation, a MKT technique represents each of the targetregions of the reference image I_(r) with a kernel-weighted histogram q,e.g. of the pixel intensity values contained in the target region. Thehistogram q comprises a plurality of histogram bins, i.e. q=(q₁, q₂, . .. , q_(m))^(T), where m is the number of bins in the histogram. The binsof the histogram are weighted with a kernel function centred at positionc in the reference image I_(r) which corresponds to the position of oneof the predetermined set of points 504. In the same way for a candidateregion 506 of the image I_(i), a kernel-weighted histogram p(c′) isdetermined with the kernel function centred at position c′ in the imageI_(i). It is assumed that the position c′ is close to the position c,and the difference between c and c′ can be expressed as Δc=c′−c. Asimilarity function between the two histograms q(c) and p(c′) can beused to find a value for Δc which provides an improved correspondencebetween the target region of the reference image I_(r) and the candidateregion 506 of the image I_(i). This method can be iterated until thevalue of Δc falls below a threshold or until a maximum number ofiterations have been performed. This idea can be expanded to multiplekernels such that a transformation ΔC can be found which provides a goodcorrespondence for tracking multiple target regions of the referenceimage I_(r) to the candidate regions 506 of the image I_(i). With singlekernel tracking, Δc can be found as a translation, i.e. Δc=(Δc_(x),Δc_(y)); but with multiple kernel tracking, ΔC can be found as a morecomplex transformation, e.g. an affine transformation which includesrotations (θ) and/or scaling (λ) functions, i.e. ΔC=(Δc_(x), Δc_(y), θ,λ). Therefore, in summary, the MKT logic 116 implements the MKTtechnique by iteratively optimizing the similarity between featurehistograms (e.g. intensity histograms) of the set of target regions ofthe reference image and corresponding feature histograms of the set ofcandidate regions by iteratively varying the positions of the candidateregions.

Some of the candidate regions 506 of the image I_(i) may be rejected ifthey are determined to be too flat for the LKI algorithm to work with.The LKI algorithm relies on gradients in order to converge to asolution, so if image regions are flat the LKI algorithm does not alwaysprovide good results. The MKT technique can provide a simple way ofdetermining whether a region is flat, such that a point in a flat regioncan be rejected, such that it is not used by the LKI logic 118. Forexample, a counter (or “weight”) for a region can be used as anindication as to whether the region is flat. Reading the histogram fromleft to right, if a bin is not zero its weight is incremented. If, inaddition, the bin to the left of the current bin is zero then the weightof the current bin is incremented by another 1. If the sum of all theweights is greater than 3 then the region is used for tracking in theLKI algorithm. Otherwise the region is discarded because it isdetermined to be flat. If the weight is lower than 4 it means that thepatch has constant colour, so that, it has a high probability of being aflat region. The reasoning for this is that if a region of the image hasconstant colour (i.e. it is a flat region), this leads to a histogramwith a single non-zero bin, because all the pixels have the same value.A flat region can be altered by noise and the quantization of its values(when generating the histogram) which can lead to histograms with twoconsecutive non-zero bins for flat regions. For a region to beconsidered non-flat, its histogram should have at least twonon-consecutive non-zero bins (so the colours in the region are moredifferent than colours altered by noise) or three consecutive non-zerobins. The algorithm of this methodology can be seen below:

-   -   weight=0    -   for each bin ‘i’ in the histogram        -   if the bin(i) !=0 then            -   weight=weight+1            -   if the bin(i−1)==0 then weight=weight+1    -   enf for    -   if weight>3 then use the point to track

FIG. 5 shows the regions which are not determined to be too flat assquares, e.g. square 508. Some of the regions which are determined bythe MKT logic 116 are not provided to the LKI logic 118 because they aretoo flat and as such FIG. 5 shows that some regions (e.g. region 506 ₁₃)do not have an associated square meaning that they are not used by theLKI logic 118.

In step S404 the LKI logic 118 uses the positions of at least some ofthe set of candidate regions determined by the MKT technique toinitialize the LKI algorithm. The LKI algorithm is known in the art, forexample as described in “Lucas-Kanade 20 Years On: A Unifying Framework”by Simon Baker and Iain Matthews, International Journal of ComputerVision, 2004, pp221-255.

In step S406 the LKI logic 118 uses the LKI algorithm to determine a setof points of the image I_(i) which correspond to at least some of thepoints of the predetermined set of points of the reference image I_(r).Since the LKI algorithm is known in the art, for conciseness, an indepth explanation of the LKI algorithm is not provided herein. However,as a higher-level explanation, the LKI algorithm aims to minimise thesum of squared error between two image patches: a first patch being atarget region of the reference image I_(r) and the second patch being acandidate region of the image I_(i) which is warped back onto thecoordinates of the reference image. The sum of squared error between thetwo image patches is minimised by varying the warping parameter p (i.e.changing p to p+Δp) to find a value for Δp which minimises the sum ofsquared error. According to the LKI algorithm this is done iterativelyuntil the value of Δp is below a threshold or until a maximum number ofiterations have been performed. The final value of the warping parameterp after the LKI algorithm has been performed is used to determine thepositions of a set of points in the image I_(i) which correspond to atleast some of the predetermined set of points of the reference imageI_(r).

A problem which the LKI algorithm can sometimes encounter is related tothe image gradients in the reference image I_(r). When a gradient in onedirection dominates the gradient in the perpendicular direction (e.g.when the gradient on the x axis ∇_(x)I dominates the gradient on the yaxis ∇_(y)I, or vice-versa), the results of the LKI algorithm may beerroneous. However, as can be seen in the more detailed description ofthe LKI algorithm provided below, for each of the points of the set ofpoints of the image I_(i) which aren't determined to be too flat, theLKI algorithm includes determining a warped version of an image patchsurrounding the point, and determining a Hessian matrix for the imagepatch. The elements of the Hessian matrix indicate sums of squaredvalues of the gradients in different directions across the warpedversion of the image patch. The problem of gradients in one directiondominating gradients in another direction can be addressed by comparingthe sum of the squared values of the gradients on x and y axes. If thesum of the squared values of the gradients for a region in one directionis at least 20 times bigger than in the perpendicular direction then theregion is discarded. By discarding a region in this way, the LKI logic118 will not output a point correspondence for the discarded region. Itis noted that this comparison does not significantly add to thecomputation performed by the point correspondence logic 110 because thesum of the squared values of the gradients can be extracted from theHessian matrix (which is computed as part of the LKI algorithm). TheHessian is referred to as “ill-conditioned” when the ratio between thetwo gradients is large (e.g. ≥20).

FIG. 5 shows regions which have ill-conditioned Hessians with squareshaving dashed lines, such as region 508 ₅₂. The point correspondencesdetermined by the point correspondence logic 110 are provided to thetransformation logic 112. In the example shown in FIG. 5 there are 35regions determined by the MKT logic 116 surrounding the respective 35predetermined points 504. Seven of those regions are flat (and do nothave corresponding squares shown in FIG. 5) and as such the MKT logic116 discards them. Of the remaining 28 regions, the LKI logic 118determines that five of them have ill conditioned Hessians (and havesquares shown with dashed lines in FIG. 5) and as such the LKI logic 118discards them. Therefore the point correspondences are determined forthe remaining 23 regions (i.e. those regions shown with solid linesquares in FIG. 5) and the point correspondences for these regions areprovided to the transformation logic 112.

In step S212 the transformation logic 112 determines parameters of atransformation to be applied to the image I_(i) based on an error metricwhich is indicative of an error between a transformation of the set ofpoints received from the point correspondence logic 110 and thecorresponding points of the set of predetermined set of points of thereference image I_(r).

For example, the transformation for image I_(i) is a homography which isdescribed by a matrix H_(i) which can be used to more closely align thepixel positions of the image I_(i) with the corresponding pixelpositions of the reference image I_(r). As an example, the homographymay be restricted to be a 2D projective transformation. This provides agood trade-off between flexibility and simplicity of the alignmentestimation. The step of determining parameters of the transformation maycomprise determining the elements of the homography matrix, H_(i), suchthat:

x_(i)=H_(i)x_(r),

where x_(i) is the set of points of the image I_(i) which correspond tothe points x_(r) of the reference image, as determined by the pointcorrespondence logic 110.

Step S212 comprises optimizing the elements of the homography matrix,H_(i), by computing the Minimum Mean Squared Error (MMSE) over the twosets of points, x_(i) and x_(r). This comprises finding values for theelements of the matrix H_(i) which provide the minimum mean squarederror for the set of points, e.g. by solving the equation:

${{\begin{pmatrix}0 & 0 & 0 & {- x_{r}^{j}} & {- y_{r}^{j}} & 1 & {x_{r}^{j}y_{i}^{j}} & {y_{r}^{j}y_{i}^{j}} \\x_{r}^{j} & y_{r}^{j} & 1 & 0 & 0 & 0 & {{- x_{r}^{j}}x_{i}^{j}} & {{- y_{r}^{j}}x_{i}^{j}}\end{pmatrix}\begin{pmatrix}h_{1} \\h_{2} \\h_{3} \\h_{4} \\h_{5} \\h_{6} \\h_{7} \\h_{8}\end{pmatrix}} = \begin{pmatrix}{- y_{i}^{j}} \\x_{i}^{j}\end{pmatrix}},$

for j=0 to N where N is the number of points for which correspondencesare determined. It is noted that N is at least four so that a solutioncan be found for H_(i) and in the example described above with referenceto FIG. 5, N=23. Usually, increasing N would increase the accuracy ofthe values determined for the matrix H_(i). To arrive at the equationabove, it is noted that

${H_{i} = \begin{pmatrix}h_{1} & h_{2} & h_{3} \\h_{4} & h_{5} & h_{6} \\h_{7} & h_{8} & 1\end{pmatrix}},{x_{r}^{j} = \left( {x_{r}^{j},y_{r}^{j}} \right)}$

for the j^(th) point of the reference image I_(r) and x_(i) ^(j)=(x_(i)^(j), y_(i) ^(j)) for the j^(th) point of the image I_(i).

In other examples, other error metrics (other than the MMSE) may be usedto find a solution for the matrix H_(i).

In step S214 the transformation logic 112 applies the transformation tothe image I_(i) to bring it closer to alignment with the reference imageI_(r). The alignment logic 104 performs steps S210 to S214 for each ofthe images that are received from the selection logic 102 except for thereference image (there is no need to transform the reference image),such that a respective transformation is applied to the differentimages.

Steps S210, S212 and S214 could be implemented as a stand-alone methodfor transforming a first image (e.g. an image I_(i)) to bring it closerto alignment with a second image (e.g. the reference image I_(r)). Thesesteps are described herein in the context of part of the noise reductionmethod shown in FIG. 2, but they could be used in other scenarios inwhich it would be useful to transform a first image such that it moreclosely aligns with a second image.

Even though the images have been transformed, there may still exist somemisalignment between the images and the reference image. Misalignmentbetween the images may be detrimental when the images are combined.Therefore if a transformed image is significantly misaligned with thereference image then that transformed image may be discarded by thealignment logic 104, as described below in steps S216 and S218.

In step S216 the alignment measuring logic 114 determines measures ofalignment of the respective transformed images with the reference image.The transformed images are denoted W_(i). As an example, the measure ofalignment of a transformed image W_(i) is a misalignment parameterτ_(i), which may for example be determined as the sum (over all of thepixel positions (x,y) of the image) of the absolute differences betweenthe transformed image W_(i)(x, y) and the reference image I_(r)(x, y).That is:

τ_(i)=Σ_(xy) |W _(i)(x, y)−I _(r)(x, y)|.

In step S218 the alignment measuring logic 114 determines, for each ofthe transformed images, whether the respective measure of alignmentindicates that the alignment of the transformed image W_(i) with thereference image I_(r) is below a threshold alignment level. Independence thereon, the alignment measuring logic 114 selectivelydiscards images which are determined to be misaligned. Images which arediscarded are not provided from the alignment logic 104 to the combininglogic 106. In the example in which the measure of alignment of an imageI_(i) is a misalignment parameter τ_(i), an image may be discarded ifthe misalignment parameter τ_(i) is above a threshold. As an example,the threshold may depend on the mean of the misalignment parameters,μ(τ), for the different images and on the standard deviation of themisalignment parameters, σ(τ), for the different images, where τrepresents all of the misalignment parameters for the different images,i.e. τ={τ₁, . . . , τ_(N)}, where N is the number of different imagesfor which a misalignment parameter is determined. For example, thethreshold may be μ(τ)+ε₂σ(τ) (where as an example ε₂ may be in the range1.2≤ε₂≤1.5). A hugely misaligned image may adversely affect thethreshold, so in another example, rather than using a threshold todiscard misaligned images, a predetermined number of the best alignedimages (i.e. those images with the lowest misalignment parameters τ_(i))may be selected for use, and the other images may be discarded.

As an example, FIG. 6 shows a graph of the misalignment parameters 602for a set of images. The images 6 and 7 were discarded by the selectionlogic 102 because they were too blurry and so misalignment parametersare not calculated for those images. Image number 5 is the referenceimage and as such its misalignment parameter is zero. The dashed line604 represents the misalignment threshold (e.g. set at μ(τ)+ε₂σ(τ)). Itcan be seen in this example that the misalignment parameters for images0 and 8 are above the misalignment threshold 604, whereas themisalignment parameters for images 1 to 4 and 9 are below themisalignment threshold 604.

In step S218 the alignment measuring logic 114 discards misalignedimages, i.e. images for which the misalignment parameter is above themisalignment threshold. This corresponds to discarding images if theirmeasures of alignment are below a threshold alignment level.

Therefore, in the example described above, in step S218 an image I_(i)with a misalignment parameter τ_(i) is discarded if τ_(i)>μ(τ)+ε₂σ(τ).It is noted that in some other examples step S218 might not beperformed. That is, in some examples, images are not discarded based ontheir alignment with the reference image. This may help to simplify theprocess, but may result in misalignment artefacts appearing in the finalimage.

Images which pass the alignment test are passed from the alignment logic104 to the combining logic 106. Conversely, images which are discardedby the alignment logic 104 are not passed from the alignment logic 104to the combining logic 106.

The combining logic 106 operates to combine the transformed images itreceives from the alignment logic 104. In order to do this, in step S220the combining logic 106 determines weights for the transformed imagesusing the measures of alignment determined by the alignment measuringlogic 114. Then in step S222 the combining logic 106 combines aplurality of images including the transformed images received from thealignment logic 104 using the determined weights to form a reduced noiseimage. The plurality of images which are combined in step S222 may ormay not include the reference image. In preferred examples describedherein the plurality of images which are combined in step S222 includesthe reference image, which is the sharpest of the images. In otherexamples, e.g. if the reference image is selected differently, e.g. asthe temporally middle image, then it may be beneficial to leave thereference image out of the group of images which are combined in stepS222, e.g. if the reference image is particularly blurry. Selecting thetemporally middle image as the reference image may sometimes be asuitable choice since it is likely that, on average, the images will becloser to alignment with the temporally middle image than to a differentimage. Furthermore, selecting the temporally middle image as thereference image would avoid the processing needed to determine thesharpness of the images in order to select the reference image. In theseexamples, the other images are aligned to the reference image and thensome of the aligned images (which might not include the reference image)are combined to form the reduced noise image in step S222.

As an example, the images may be combined using a bilateral filter withweights for each pixel of each image defined in dependence on themisalignment parameter of the image, τ_(i), and the difference in pixelvalue between the pixel of the image and the corresponding pixel of thereference image. The resultant image is the accumulation of thetransformed images after weighting each pixel with the appropriateweight. For example, the images may be ordered depending on theiralignment with the reference image, e.g. by ordering the images usingthe misalignment parameters to form an ordered set of images. An indexvalue, i, indicates the position of an image in the ordered set. A lowindex value, i, is given to a highly aligned image (i.e. an image with alow misalignment parameter, τ_(i)), whereas a higher index value, i, isgiven to a less aligned image (i.e. an image with a higher misalignmentparameter, τ_(i)). For example, if there are N images, an index value ofi=1 is given to the best aligned image (i.e. the image with the lowestmisalignment parameter, τ_(i)), and an index value of i=N is given tothe worst aligned image (i.e. the image with the highest misalignmentparameter, τ_(i)). For example, a transformed image W_(i) has red, greenand blue pixel values at a pixel position (x, y), denoted respectivelyas W_(i) ^(R)(x, y), W_(i) ^(G)(x, y) and W_(i) ^(B)(x, y). Similarly,the reference image I_(r) has red, green and blue pixel values at apixel position (x, y), denoted respectively as I_(r) ^(R)(x, y), I_(r)^(G)(x, y) and I_(r) ^(B)(x, y). As an example, the weight, ω_(i)(x, y),for a pixel at position (x, y) of the transformed image W_(i) isdetermined according to the equation:

${\omega_{i}\left( {x,y} \right)} = {\frac{1}{\sum_{j}{\omega_{j}\left( {x,y} \right)}}e^{- \frac{i^{2}}{2\sigma_{\tau}^{2}}}e^{- \frac{{({{W_{i}^{R}{({x,y})}} - {I_{r}^{R}{({x,y})}}})}^{2} + {({{W_{i}^{G}{({x,y})}} - {I_{r}^{G}{({x,y})}}})}^{2} + {({{W_{i}^{B}{({x,y})}} - {I_{r}^{B}{({x,y})}}})}^{2}}{2\sigma_{diff}^{2}}}}$

where σ_(τ) is the standard deviation used to define the 0 mean Gaussianof the misalignment weighting (this is a parameter that can be tuned,and as an example may be equal to 6); and σ_(diff) is the standarddeviation used to define the 0 mean Gaussian of the pixel difference(this is a parameter that can also be tuned, and as an example may beequal to 20). The factor of

$\frac{1}{\sum_{j}{\omega_{j}\left( {x,y} \right)}}$

is a normalization factor which means that for each pixel position (x,y) the weights of the different images sum to one.

It can be appreciated that since the weights depend upon the alignmentof the image with the reference image, the resulting combined pixelvalues are weighted in favour of images which are closely aligned withthe reference image. This reduces artifacts which may occur due tomisalignment between the images which are combined.

The reduced noise image (which is denoted S′ herein) is output from thecombining logic 106. In some examples, this could be the end of thenoise reduction process and the reduced noise image could be provided asthe output of the processing module 100. However, in other examples,some motion correction may be applied to the reduced noise image beforeit is outputted from the processing module 100. Motion correction may bebeneficial because when the captured scene has regions with motion thenthe combined image S′ may contain artifacts due to the motion in thescene (and/or motion of the camera) between the times at which differentones of the combined images are captured.

As an example, the reduced noise image output from the combining logic106 may be received by the motion correction logic 108, and in step S224the motion correction logic 108 determines motion indications indicatinglevels of motion for areas of the reduced noise image, S′. In examplesdescribed herein, this is done by first determining a “backgroundimage”, B, which has pixel values corresponding to an average (e.g. meanor median) of the corresponding pixel values of the transformed imagesW_(i) and optionally the reference image I_(r), determined pixel bypixel. The background image, B, may be a downscaled version of theimages. For example, the original images may comprise 1440×1080 pixelsand the downscaled background image may comprise 256×192 pixels. Thesenumbers are just given by way of example. Downscaling processes areknown in the art to convert images between different resolutions oraspect ratios, etc. Downscaling the background image reduces the numberof pixels in the background image and therefore reduces the amount ofcomputation that is performed on the background image, withoutsignificantly affecting the result of the motion correction.

FIG. 7a shows an example of a background image 702. There is some motionin the scene between the times at which the different images arecaptured and, as such, parts of the background image 702 are blurred.

A binary motion mask can then be determined which indicates for eachpixel of the background image whether or not there is motion. Forexample, the binary value of the motion mask mask(x, y) at the pixelposition (x, y) can be determined according to the equation:

mask(x, y)=|I _(r) ^(R)(x, y)−B ^(R)(x, y)|>λ∨|I _(r) ^(G)(x, y)−B^(G)(x, y)|>λ∨|I _(r) ^(B)(x, y)−B ^(B)(x, y)|>λ

where B^(B)(x, y), B^(G)(x, y) and B^(B)(x, y) are the red green andblue components of the background image at pixel position (x, y), λ is athreshold parameter which may for example be set to 8, and ∨ is an ORoperator. So if any of the colour components of the background imagediffer from the corresponding colour components of the reference imageby more than the threshold parameter then the mask(x, y) value is set to1 to indicate that there is motion at the pixel position (x, y),otherwise the mask(x, y) value is set to 0 to indicate that there is notmotion at the pixel position (x, y).

FIG. 7b shows the motion mask 704 for the background image shown in FIG.7a . In FIG. 7b a pixel is white if the motion mask at that positionindicates that there is motion in the background image (e.g. if mask(x,y)=1), and a pixel is black if the motion mask at that positionindicates that there is not motion in the background image (e.g. ifmask(x, y)=0)

It can be seen in FIG. 7b that the binary motion mask includes a lot ofsmall regions which appear to be indicative of motion but when comparedto the image 702 it can be seen that these small regions often do notrelate to significant motion in the scene. Therefore, the binary motionmask may be cleaned using a set of morphological operations, e.g.consisting of two erosion operations followed by two dilatationoperations.

The cleaned motion mask 706 is shown in FIG. 7c . It can be appreciatedthat the white areas in FIG. 7c correspond closely to areas of motion inthe image 702.

The motion mask 706 is smoothed in order to smooth transitions betweenblack and white areas of the mask. In order to smooth the mask 706, themask 706 may be convolved using a Gaussian filter. The resultingsmoothed mask 708 is shown in FIG. 7d . The smoothed mask 708 is notrestricted to binary values and may include values between 0 and 1.

Then the smoothed motion mask 708 is upscaled to match the resolution ofthe original images (e.g. 1440×1080 pixels). Methods of upscaling areknown in the art. In step S226, the motion correction logic 108 combinesthe reference image I_(r)(x, y) and the reduced noise image S′(x, y)using the upscaled smoothed motion mask (denoted MASK(x, y)) to form amotion-corrected reduced noise image S″(x, y). In this way, areas of thereduced noise image S′ are mixed with corresponding areas of thereference image I_(r) based on the motion mask MASK(x, y), e.g.according to the equation:

S″(x, y)=I _(r)(x, y)*MASK(x, y)+S′(x, y)*(1−MASK(x, y))

Furthermore, in some examples, a spatial bilateral filter may be appliedto those regions which are taken from the reference picture. That is,the reference image, I_(r)(x, y), may be spatially filtered before usingit to determine the motion-corrected reduced noise image S″(x, y)according to the equation given above.

In step S228 the motion-corrected, reduced noise image S″ is outputtedfrom the processing module 100 as the result of the method. The image S″may subsequently be used for any suitable purpose, e.g. it may be storedin a memory or used by some other processing module or displayed on adisplay.

FIG. 8 shows a reference image (I_(r)) 802, a reduced noise image (S′)804 and a motion-corrected reduced noise image (S″) 806 in one example.The amount of random noise in static regions of the images 804 and 806(e.g. on the white wall of the background in the image) is less than therandom noise in the corresponding region of the reference image 802. Theimage 804 exhibits some motion artifacts, for example the bin and theleg seem to blur together in image 804. These motion artifacts have beencorrected in image 806.

The set of images in the examples described above may comprise aplurality of images captured in a burst mode. Alternatively the set ofimages may comprise a plurality of frames of a video sequence. Whenworking with videos, the method may have a few variations. With a set ofvideo frames, it is the most recent (i.e. the last frame) to which thedenoising is applied, and the previous n frames are used to denoise thelast frame. The number, n, can vary depending on the needs orcapabilities of the hardware. In this case the last frame of the videosequence may be used as a reference image, and hence it is not necessaryto select a reference image and discard blurry images. In addition thealignment step may be performed using a plurality of n previous framesand it is computed incrementally, such that aligning the frame n−2 usesthe output of the alignment of the frame n−1, and so on. Since a videosequence may contain sudden scene changes (which may be referred to as“cuts”), it may be important to detect the scene changes after aligningthe images. A cut detector may be implemented based on generating a 3Dhistogram of 8 bins for each channel (red, green and blue), giving atotal of 512 bins. The histogram of a current frame (hist_(i)(r, g, b))is compared with a histogram the previous frame (hist_(i−1)(r, g, b))and a cut is detected if the sum of the absolute differences of all binsdivided by the number of pixels (N) is greater than a threshold, ε₃,where as an example the threshold may be in the range 0.02≤ε₃≤0.1. Thatis, a cut may be detected when the following equation is satisfied:

${\frac{1}{N}{\sum\limits_{r}^{8}{\sum\limits_{g}^{8}{\sum\limits_{b}^{8}{{{{hist}_{i}\left( {r,g,b} \right)} - {{hist}_{i - 1}\left( {r,g,b} \right)}}}}}}} > ɛ_{3}$

In some examples, rather than determining the histograms hist_(i)(r, g,b) and hist_(i−1)(r, g, b) using the reference image I_(r)(x, y), thehistograms may be determined using the previously computed backgroundimage (B) because this is a small (i.e. downscaled) image, e.g. formedby computing the average of the aligned images and then downscaling.

The processing module 100 described above can be implemented in acomputer system. The computing system could be implemented in a camera,smartphone, tablet or any other suitable computing device. For example,FIG. 9 shows a computer system which comprises a GPU 902, a CPU 904 anda memory 906. The computer system also comprises other devices 908, suchas a display 910, speakers 912, a camera 914 and a keypad 916. Thecomponents of the computer system can communicate with each other via acommunications bus 918. The processing module 100 may be implemented onthe GPU 902 as shown in FIG. 9 in hardware or software or a combinationthereof. For example, if the logic blocks (102, 104, 106 and 108) of theprocessing module 100 are implemented in hardware they may be formed asparticular arrangements of transistors and other hardware componentssuited for performing the desired functions of the logic blocks asdescribed herein. In contrast, if the logic blocks (102, 104, 106 and108) of the processing module 100 are implemented in software they maycomprise sets of computer instructions which can be stored in the memory906 and can be provided to the GPU 902 for execution thereon. In otherexamples the processing module 100 could be implemented on the CPU 904.The set of images are received at the processing module 100, e.g. fromthe camera 914, and the processing module 100 outputs themotion-correction reduced noise image, which may then, for example, bedisplayed on the display 910 and/or stored in the memory 906.

Generally, any of the functions, methods, techniques or componentsdescribed above (e.g. the processing module 100 and its components) canbe implemented in modules using software, firmware, hardware (e.g.,fixed logic circuitry), or any combination of these implementations. Theterms “module,” “functionality,” “component”, “block”, “unit” and“logic” are used herein to generally represent software, firmware,hardware, or any combination thereof.

In the case of a software implementation, the module, functionality,component, unit or logic represents program code that performs specifiedtasks when executed on a processor (e.g. one or more CPUs). In oneexample, the methods described may be performed by a computer configuredwith software in machine readable form stored on a computer-readablemedium. One such configuration of a computer-readable medium is signalbearing medium and thus is configured to transmit the instructions (e.g.as a carrier wave) to the computing device, such as via a network. Thecomputer-readable medium may also be configured as a non-transitorycomputer-readable storage medium and thus is not a signal bearingmedium. Examples of a computer-readable storage medium include arandom-access memory (RAM), read-only memory (ROM), an optical disc,flash memory, hard disk memory, and other memory devices that may usemagnetic, optical, and other techniques to store instructions or otherdata and that can be accessed by a machine.

The software may be in the form of a computer program comprisingcomputer program code for configuring a computer to perform theconstituent portions of described methods or in the form of a computerprogram comprising computer program code means adapted to perform allthe steps of any of the methods described herein when the program is runon a computer and where the computer program may be embodied on acomputer readable medium. The program code can be stored in one or morecomputer readable media. The features of the techniques described hereinare platform-independent, meaning that the techniques may be implementedon a variety of computing platforms having a variety of processors.

Those skilled in the art will also realize that all, or a portion of thefunctionality, techniques or methods may be carried out by a dedicatedcircuit, an application-specific integrated circuit, a programmablelogic array, a field-programmable gate array, or the like. For example,the module, functionality, component, unit or logic (e.g. the logicblocks of the processing module 100) may comprise hardware in the formof circuitry. Such circuitry may include transistors and/or otherhardware elements available in a manufacturing process. Such transistorsand/or other elements may be used to form circuitry or structures thatimplement and/or contain memory, such as registers, flip flops, orlatches, logical operators, such as Boolean operations, mathematicaloperators, such as adders, multipliers, or shifters, and interconnects,by way of example. Such elements may be provided as custom circuits orstandard cell libraries, macros, or at other levels of abstraction. Suchelements may be interconnected in a specific arrangement. The module,functionality, component, unit or logic (e.g. the logic blocks of theprocessing module 100) may include circuitry that is fixed function andcircuitry that can be programmed to perform a function or functions;such programming may be provided from a firmware or software update orcontrol mechanism. In an example, hardware logic has circuitry thatimplements a fixed function operation, state machine or process.

It is also intended to encompass software which “describes” or definesthe configuration of hardware that implements a module, functionality,component, unit or logic (e.g. the logic blocks of the processing module100) described above, such as HDL (hardware description language)software, as is used for designing integrated circuits, or forconfiguring programmable chips, to carry out desired functions. That is,there may be provided a computer readable storage medium having encodedthereon computer readable program code in the form of an integratedcircuit definition dataset that when processed in an integrated circuitmanufacturing system configures the system to manufacture a graphicsprocessing system configured to perform any of the methods describedherein, or to manufacture a graphics processing system comprising anyapparatus described herein. The IC definition dataset may be in the formof computer code, e.g. written in a suitable HDL such asregister-transfer level (RTL) code. An example of processing anintegrated circuit definition dataset at an integrated circuitmanufacturing system so as to configure the system to manufacture agraphics processing system will now be described with respect to FIG.10.

FIG. 10 shows an example of an integrated circuit (IC) manufacturingsystem 1002 which comprises a layout processing system 1004 and anintegrated circuit generation system 1006. The IC manufacturing system1002 is configured to receive an IC definition dataset (e.g. defining aprocessing module as described in any of the examples herein), processthe IC definition dataset, and generate an IC according to the ICdefinition dataset (e.g. which embodies a processing module as describedin any of the examples herein). The processing of the IC definitiondataset configures the IC manufacturing system 1002 to manufacture anintegrated circuit embodying a processing module as described in any ofthe examples herein. More specifically, the layout processing system1004 is configured to receive and process the IC definition dataset todetermine a circuit layout. Methods of determining a circuit layout froman IC definition dataset are known in the art, and for example mayinvolve synthesising RTL code to determine a gate level representationof a circuit to be generated, e.g. in terms of logical components (e.g.NAND, NOR, AND, OR, MUX and FLIP-FLOP components). A circuit layout canbe determined from the gate level representation of the circuit bydetermining positional information for the logical components. This maybe done automatically or with user involvement in order to optimise thecircuit layout. When the layout processing system 1004 has determinedthe circuit layout it may output a circuit layout definition to the ICgeneration system 1006. The IC generation system 1006 generates an ICaccording to the circuit layout definition, as is known in the art. Forexample, the IC generation system 1006 may implement a semiconductordevice fabrication process to generate the IC, which may involve amultiple-step sequence of photo lithographic and chemical processingsteps during which electronic circuits are gradually created on a wafermade of semiconducting material. The circuit layout definition may be inthe form of a mask which can be used in a lithographic process forgenerating an IC according to the circuit definition. Alternatively, thecircuit layout definition provided to the IC generation system 1006 maybe in the form of computer-readable code which the IC generation system1006 can use to form a suitable mask for use in generating an IC. Thedifferent processes performed by the IC manufacturing system 1002 may beimplemented all in one location, e.g. by one party. Alternatively, theIC manufacturing system 1002 may be a distributed system such that someof the processes may be performed at different locations, and may beperformed by different parties. For example, some of the stages of: (i)synthesising RTL code representing the IC definition dataset to form agate level representation of a circuit to be generated, (ii) generatinga circuit layout based on the gate level representation, (iii) forming amask in accordance with the circuit layout, and (iv) fabricating anintegrated circuit using the mask, may be performed in differentlocations and/or by different parties.

In other examples, processing of the integrated circuit definitiondataset at an integrated circuit manufacturing system may configure thesystem to manufacture a processing module without the IC definitiondataset being processed so as to determine a circuit layout. Forinstance, an integrated circuit definition dataset may define theconfiguration of a reconfigurable processor, such as an FPGA, and theprocessing of that dataset may configure an IC manufacturing system togenerate a reconfigurable processor having that defined configuration(e.g. by loading configuration data to the FPGA).

In some examples, an integrated circuit definition dataset could includesoftware which runs on hardware defined by the dataset or in combinationwith hardware defined by the dataset. In the example shown in FIG. 10,the IC generation system may further be configured by an integratedcircuit definition dataset to, on manufacturing an integrated circuit,load firmware onto that integrated circuit in accordance with programcode defined at the integrated circuit definition dataset or otherwiseprovide program code with the integrated circuit for use with theintegrated circuit.

The term ‘processor’ and ‘computer’ are used herein to refer to anydevice, or portion thereof, with processing capability such that it canexecute instructions, or a dedicated circuit capable of carrying out allor a portion of the functionality or methods, or any combinationthereof.

Although the subject matter has been described in language specific tostructural features and/or methodological acts, it is to be understoodthat the subject matter defined in the appended claims is notnecessarily limited to the specific features or acts described above.Rather, the specific features and acts described above are disclosed asexample forms of implementing the claims. It will be understood that thebenefits and advantages described above may relate to one example or mayrelate to several examples.

Any range or value given herein may be extended or altered withoutlosing the effect sought, as will be apparent to the skilled person. Thesteps of the methods described herein may be carried out in any suitableorder, or simultaneously where appropriate. Aspects of any of theexamples described above may be combined with aspects of any of theother examples described to form further examples without losing theeffect sought.

What is claimed is:
 1. A method of forming a reduced noise image using aset of images, the method comprising: obtaining a plurality oftransformed images by applying respective transformations to at leastsome of the images of the set to bring them closer to alignment with areference image from the set of images; determining, for each of thetransformed images, a respective measure of alignment of thattransformed image with the reference image, wherein the measure ofalignment for a transformed image is a misalignment parameter τ_(i),determined as the sum, over all of the pixel positions (x,y) of thetransformed image, of the absolute differences between the transformedimage W_(i)(x, y) and the reference image I_(r)(x, y); determiningweights for one or more of the transformed images using the determinedmeasures of alignment; and combining a plurality of images includingsaid one or more of the transformed images using the determined weightsto form a reduced noise image which has less noise than the referenceimage.
 2. The method of claim 1, further comprising selecting one of theimages of the set of images to be the reference image by: determiningsharpness indications for the images of the set of images; and based onthe determined sharpness indications, selecting the sharpest image fromthe set of images to be the reference image.
 3. The method of claim 2,further comprising discarding an image if the determined sharpnessindication for the image is below a sharpness threshold.
 4. The methodof claim 2, wherein the sharpness indications are sums of absolutevalues of image Laplacian estimates for the respective images.
 5. Themethod of claim 1, further comprising, for each of the transformedimages, determining whether the respective measure of alignmentindicates that the alignment of the transformed image with the referenceimage is below a threshold alignment level, and in dependence thereonselectively including the transformed image as one of said one or moreof the transformed images for which weights are determined.
 6. Themethod of claim 1, wherein either: said plurality of images which arecombined to form the reduced noise image further includes the referenceimage; or said plurality of images which are combined to form thereduced noise image does not include the reference image.
 7. The methodof claim 1, further comprising determining the transformations to applyto said at least some of the images, wherein for each of said at leastsome of the images the respective transformation is determined by:determining a set of points of the image which correspond to apredetermined set of points of the reference image; and determiningparameters of the transformation for the image based on an error metricwhich is indicative of an error between a transformation of at leastsome of the determined set of points of the image and the correspondingpoints of the predetermined set of points of the reference image.
 8. Themethod of claim 7, wherein the set of points of the image are determinedusing the Lucas Kanade Inverse algorithm, and wherein the Lucas KanadeInverse algorithm is initialized using the results of a multiple kerneltracking technique.
 9. The method of claim 8, wherein the multiplekernel tracking technique determines the positions of a set of candidateregions based on a similarity between a set of target regions and theset of candidate regions, wherein the target regions are respectivelypositioned over the positions of the predetermined set of points of thereference image, and wherein the determined positions of the set ofcandidate regions are used to initialize the Lucas Kanade Inversealgorithm.
 10. The method of claim 1, wherein the set of imagescomprises a plurality of images captured in a burst mode.
 11. The methodof claim 1, wherein the set of images comprises a plurality of frames ofa video sequence.
 12. A processing module for forming a reduced noiseimage using a set of images, the processing module comprising: alignmentlogic configured to: apply respective transformations to at least someof the images of the set to bring them closer to alignment with thereference image from the set of images; and determine, for each of thetransformed images, a respective measure of alignment of thattransformed image with the reference image, wherein the measure ofalignment for a transformed image is a misalignment parameter τ_(i)determined as the sum, over all of the pixel positions (x,y) of thetransformed image, of the absolute differences between the transformedimage W_(i)(x, y) and the reference image I_(r)(x, y); and combininglogic configured to: determine weights for one or more of thetransformed images using the determined measures of alignment; andcombine a plurality of images including said one or more of thetransformed images using the determined weights to form a reduced noiseimage which has less noise than the reference image.
 13. The processingmodule of claim 12, further comprising selection logic configured toselect one of the images of the set of images to be the reference image.14. The processing module of claim 13, wherein the selection logic isconfigured to select one of the images of the set of images to be thereference image by: determining sharpness indications for the images ofthe set of images; and based on the determined sharpness indications,selecting the sharpest image from the set of images to be the referenceimage.
 15. The processing module of claim 14, wherein the selectionlogic is further configured to discard an image such that it is notprovided to the alignment logic if the determined sharpness indicationfor the image is below a sharpness threshold.
 16. The processing moduleof claim 14, wherein the sharpness indications are sums of absolutevalues of image Laplacian estimates for the respective images.
 17. Theprocessing module of claim 12, wherein the alignment logic is furtherconfigured to, for each of the transformed images, determine whether therespective measure of alignment indicates that the alignment of thetransformed image with the reference image is below a thresholdalignment level, and in dependence thereon selectively include thetransformed image as one of said one or more of the transformed imagesto be passed to the combining logic for which weights are to bedetermined.
 18. The processing module of claim 12, further comprisingmotion correction logic configured to apply motion correction to thereduced noise image formed by the combining logic.
 19. The processingmodule of claim 18, wherein the motion correction logic is configured toapply motion correction to the reduced noise image by: determiningmotion indications indicating levels of motion for areas of the reducednoise image; and mixing areas of the reduced noise image withcorresponding areas of the reference image based on the motionindications to form a motion-corrected, reduced noise image.
 20. Anon-transitory computer readable storage medium having stored thereonprocessor executable instructions that when executed cause at least oneprocessor to form a reduced noise image using a set of images, theforming a reduced noise image comprising: applying respectivetransformations to at least some of the images of the set to bring themcloser to alignment with a reference image from the set of images;determining, for each of the transformed images, a respective measure ofalignment of that transformed image with the reference image, whereinthe measure of alignment for a transformed image is a misalignmentparameter τ_(i), determined as the sum, over all of the pixel positions(x,y) of the transformed image, of the absolute differences between thetransformed image W_(i)(x, y) and the reference image I_(r)(x, y);determining weights for one or more of the transformed images using thedetermined measures of alignment; and combining a plurality of imagesincluding said one or more of the transformed images using thedetermined weights to form a reduced noise image which has less noisethan the reference image.